The present invention relates to the preparation of coated abrasive materials comprising a curable binder and specifically to formulations for finishing fabric substrates that are curable by radiation.
The use of radiation curable formulations in the preparation of coated abrasives has been taught for many years. One of the earliest examples of this form of binder is described in U.S. Pat. No. 4,547,204 which describes radiation curing urethane acrylates and epoxy acrylates using electron beam radiation. In addition U.S. Pat. No. 4,773,920 taught the use of binder/grain mixtures curable by radiation-induced free radical polymerization. In U.S. Pat. No. 5,014,468 the problems of UV radiation-induced polymerization are reviewed in the context of coated abrasives. It is pointed out that, in view of the limited penetration of the UV light into a formulation that comprises pigment and/or relatively coarse abrasive particles, UV radiation is somewhat limited in its utility to relatively thin layers. On the other hand, electron beam induced polymerization can induce deterioration in some substrates if they are exposed to high dose levels, (10 to 12 Mrads) in an effort to cure thick coatings or coatings with a high level of abrasive or filler.
The problems limiting the applicability of radiation-cured polymers in coated abrasives are experienced at their most intense in finishing formulations. These are formulations added to fabric materials to prepare them to receive maker coats in the preparation of coated abrasives. Typically they comprise polymers and fillers intended to saturate the backing and provide a surface to which the maker coat will bond tightly. If the backing is too porous much of the maker will be absorbed into the body of the backing rendering it useless for the purpose of anchoring the abrasive grain when this is applied. Hence binders with a very significant amounts of filler are typically used. The filler is a necessary component to reduce the cost, block the passages within the fabric to reduce its porosity and to modify the physical properties of the backing.
In particular, the addition of filler improves the modulus of the cured formulation and at the same time reduces the amount of the (usually expensive) polymer-forming components that comprise the binder. Thus fillers are often preferred components also of maker and size coats.
The presence of heavy filler loadings is very unfavorable to the use of UV-radiation curable binders because, unless fillers are used that are transparent to the UV radiation, the UV radiation cannot penetrate far enough because of the shadowing effect of the filler particles. Electron beam radiation is effective but if a high dosage is required to penetrate the formulation, there is a strong risk of damage to the substrate.
Thus in spite of the obvious advantages of radiation-curable formulations in terms of speed of cure, such products have had difficulty moving out of the niche markets to which they are particularly well adapted, such as fining pads and abrasive discs for optical applications.
A method has now been found for curing finishes applied to a fabric using a radiation treatment so as to effect rapid cure of the finish without significant damage to the substrate fabric. This process significantly speeds up the production of coated abrasives which, using conventional techniques, is fraught with delays imposed by the need for gradual and cautious cure of the binders currently used at all stages of the production.
The present invention provides a formulation suitable for use in filling a backing material or substrate for use in the production of a coated abrasive.
The formulation includes a binder that is radiation-curable and produces adequate cure in a relatively short time despite the presence of significant amounts of fillers and/or pigments.